Yea if the pool isn't sending you work fast enough.  Also check the difficulty.  I've never used Slush so I don't know their difficulty but 128 is optimal for 180 GH/s.  64 or 32 probably won't hurt too much but if you're getting difficulty 1 shares I could see that slowing you down.  In Antminer S1's you can see difficulty under miner status in the "diff" column under pools.